The effects of amytal, amphetamine, mephenesin, and LSD were compared in 9 cases of catatonic stupor. Accessibility was increased by amytal and decreased by amphetamine. Mephenesin reduced muscle tone without affecting accessibility. The response to LSD was characterized by unmotivated laughter and crying, bizarre uncontrolled behaviour and an apparent activiation of hallucinatory and delusional material. Reference is also made to findings previously reported.